Mr. GMI don't normally post about projects while they're still in the research & gathering materials phase, but I *think* I can create a dial-up setup on my home server with a PCIe modem providing the connection on the server. Dialtone will be provided by a Cisco SPA112 ATA thing, set up so that calling some arbitrary number dials from one port to the other.
The client dialing in will be my old Dreamcast, but if this works, any client should work..!